1. Field of the Invention
The present invention relates to a process for preparing a positive electrode active material and to a nonaqueous secondary battery utilizing the same. More specifically, the invention relates to a process for preparing a positive electrode active material of lithium nickel oxide (LiNiO.sub.2), and to a nonaqueous secondary battery comprising a positive electrode containing the positive electrode active material, a negative electrode containing a lithium-containing substance or a lithium insertable and releasable substance such as carbon or graphite, and a nonaqueous ion conductor.
2. Related Art
With the size reduction and power saving of electronic devices, research and development have been made on lightweight secondary batteries which utilize an alkali metal such as lithium and are capable of discharging at a high voltage.
Where an alkali metal such as lithium is used alone for the negative electrode of a secondary battery, a dendrite (branching tree-like crystal) appears and grows on the metal surface during repeated charge and discharge cycles, i.e., during a metal dissolution and deposition process. The dendrite may penetrate through the separator of the secondary battery to come in contact with the positive electrode. This will result in a short circuit within the secondary battery. Where an alkali metal alloy is used for the negative electrode of a secondary battery instead of the alkali metal, the generation of the dendrite can be suppressed and, hence, the charge and discharge cycle characteristics are improved in comparison with the case where the alkali metal is used alone. However, the generation of the dendrite cannot be completely prevented, and there still exists the problem of the short circuit within the secondary battery.
In recent years, it has been reported that carbon materials and organic materials such as conductive polymers are useful for the negative electrode. These materials utilize a process of absorption and desorption of alkali metal ions instead of the dissolution-deposition process or the dissolution-deposition-diffusion process of a metal such as an alkali metal or its alloy. The use of these materials, in principle, prevents the generation of the dendrite which may occur where the alkali metal or its alloy is used. This remarkably reduces the risk of the short circuit within the battery. For this reason, a lithium ion battery utilizing a carbon or graphite material for its negative electrode and lithium cobaltate for its positive electrode has been put in practical use.
The use of lithium cobaltate for the positive electrode, however, entails an increase in the material cost because of the scarcity of the cobalt resource.
The use of lithium nickel oxide (LiNiO.sub.2) prepared from a nickel material which is a less expensive and more abundant resource was proposed by John Banister Goodenough et.al. (Japanese Examined Patent Publication No. 63-59507 (1988)). Lithium nickeloxide has since then been receiving attention.
Exemplary processes for preparing lithium nickel oxide (LiNiO.sub.2) are as follows:
(1) Calcining a mixture of anhydrous lithium hydroxide and art metal nickel in an atmosphere of oxygen (J. Am. Chem. Soc., 76, 1499 (1954)); PA0 (2) Mixing LiOH.H.sub.2 O and NiO, and calcining the mixture at 600.degree. C. in air, pulverizing the material mixture and then calcining again at a temperature of 600.degree. C. to 800.degree. C. for preparation of Li.sub.y Ni.sub.2-y O.sub.2 (Japanese Unexamined Patent Publication No. 2-40861 (1990)); PA0 (3) Calcining a material mixture at a temperature of 600.degree. C. to 800.degree. C. (preferably calcining the material mixture at 800.degree. C. for 6 hours twice) for preparation of LiMO.sub.2 wherein M is one or more elements selected from Co, Ni, Fe and Mn (Japanese Unexamined Patent Publication No. 4-181660 (1992)); PA0 (4) Mixing lithium peroxide (Li.sub.2 O.sub.2) and nickel oxide (NiO), then reacting the mixture at a temperature lower than 750.degree. C., and thereafter rapidly cooling the resulting product (Japanese Unexamined Patent Publication No. 5-205741 (1993)); and PA0 (5) Mixing lithium nitrate with nickel hydroxide and/or nickel oxyhydroxide, and calcining the mixture at a temperature of 500.degree. C. to 1,000.degree. C. (Japanese Unexamined Patent Publication No. 5-251079 (1993)). PA0 (1) Mixing a 4.5 mol/l lithium hydroxide aqueous solution and a 1.0 mol/l nickel nitrate aqueous solution in an equimolar mixing ratio at 60.degree. C., stirring the mixture, drying the solution under reduced pressure, pulverizing the resulting solid substance, calcining the resulting powdery substance at 300.degree. C. (preliminary calcination) and then at 800.degree. C. (main calcination) (Chemistry Express, 6, 161 (1991)); PA0 (2) Separately dissolving a nickel salt, a lithium salt and, as required, a cobalt salt in solvents, mixing the resulting solutions, and heating and calcining the mixture for preparation of LiCo.sub.x Ni.sub.1-x O.sub.2 (0.ltoreq.x.ltoreq.0.5) (Japanese Unexamined Patent Publication No. 5-325966 (1993)); and PA0 (3) Mixing an aqueous solution of a water-soluble nickel salt and an aqueous solution of a water-soluble lithium salt, drying the solution mixture, calcining the resulting cake at a temperature of 600.degree. C. to 800.degree. C. (Japanese Unexamined Patent Publication No. 6-44970 (1994)).
In these processes, a lithium compound and a nickel compound are mixed in a solid phase before the calcination and, therefore, the mixture of the nickel compound and the lithium compound is not homogeneous.
Another exemplary process (Japanese Unexamined Patent Publication No. 6-203834 (1994)) includes the steps of dissolving nickel acetate and lithium acetate in ethylene glycol under heating, further heating the solution to dryness, thermally treating the resulting solid substance at 400.degree. C. in air, pulverizing the resulting substance, and calcining the resulting powdery substance at 700.degree. C. and then at 800.degree. C. in a stream of oxygen. This process requires a prolonged reaction time and complicated preparation steps.
In view of the foregoing, processes for easy preparation of lithium nickel oxide have been proposed as follows, which are adapted to increase the homogeneity of the mixture of starting materials.
In these processes, aqueous solutions of starting materials are mixed and dried, and the resulting solid substance is calcined.
There are also known a process including the steps of mixing a powdery nickel compound slightly soluble or insoluble in water with an aqueous solution of a water-soluble lithium salt, kneading the mixture, drying the mixture, and calcining the resulting cake at a temperature of 600.degree. C. to 800.degree. C. (Japanese Unexamined Patent Publication No. 6-44971 (1994)), and a process including the steps of weighing a lithium material and a nickel material in an Li:Ni molar ratio of 1:1, mixing the lithium material and the nickel material in a small amount of water as a dispersant, drying the mixture, and calcining the resulting solid substance at 650.degree. C. in air (Japanese Unexamined Patent Publication No. 6-96769 (1994)). These processes ensure macroscopically homogenous mixing of the starting materials.
In the processes in which a lithium material and a nickel material are mixed in a solid phase and the mixture is calcined for preparation of a positive electrode active material LiNiO.sub.2 for a nonaqueous secondary battery, the nickel material and the lithium material cannot be homogeneously mixed before the calcination.
In the processes in which aqueous solutions of a lithium material and a nickel material are mixed and the mixture is dried and calcined, the homogeneity of the mixture of the nickel material and the lithium material is increased in comparison with the processes involving the solid phase mixing, because the lithium material and the nickel material are mixed in a liquid phase. However, the homogeneity of the mixture is still unsatisfactory.
The process which includes the steps of mixing nickel acetate and lithium acetate in ethylene glycol under heating, further heating the solution to dryness, thermally treating the resulting solid substance in air, pulverizing the resulting substance, and calcining twice the resulting powdery substance in a stream of oxygen presents problems of a prolonged reaction time and complicated preparation steps.
The process in which a mixture of a powdery nickel compound slightly soluble or insoluble in water and an aqueous solution of a water-soluble lithium salt is kneaded and dried, and the process in which water is used as a dispersant attempt to obtain a more homogeneous mixture. However, the resulting mixture does not have a satisfactory homogeneity after water as the solvent or dispersant is removed.